As an equipment for removing sulfur oxides from combustion exhaust gas to prevent air pollution, a wet-type limestone-gypsum desulfurization equipment is being put to practical use widely. A system of this desulfurization equipment is shown in FIG. 7. An exhaust gas 1 from a boiler, etc., is introduced from a gas entrance flue 3 into an absorbing tower 4, and by the exhaust gas 1 coming into contact with droplets of an absorbing liquid sprayed from a plurality of spray nozzles 8a disposed in each of spray headers 8 installed in multiple stages in a gas flowing direction inside the absorbing tower 4, SOx in the exhaust gas 1 are absorbed, along with soot/dust, hydrogen chloride (HCl), hydrogen fluoride (HF), and other acidic gases in the exhaust gas 1, at droplet surfaces. A mist entrained in the exhaust gas 1 is eliminated by a mist eliminator 5 installed at an exit of an absorbing tower 4, and a clean exhaust gas 2 is emitted from a chimney via an exit flue 6 and upon being reheated if necessary. A SOx concentration in the exhaust gas 1 flowing through the entrance flue 3 of the absorbing tower 4 in this process is measured by an entrance SOx meter 41.
Limestone 16, which is a SOx absorbent, is kept in a limestone slurry tank 15, and the limestone slurry is supplied by a limestone slurry pump 17 to a reservoir 4a disposed at a lower portion inside the absorbing tower 4. An amount of the limestone slurry supplied to the absorbing tower 4 is adjusted by a limestone slurry flow control valve 18 according to a SOx absorption amount inside the absorbing tower 4.
The slurry-form absorbing liquid in the reservoir 4a inside the absorbing tower 4 is pressurized by an absorbing tower circulating pump 7 and supplied via a circulation piping 25 to the spray headers 8 disposed in multiple stages in the gas flow direction at an empty tower portion at an upper portion inside the absorbing tower 4. Each spray header 8 is provided with a plurality of spray nozzles 8a, and the absorbing liquid is sprayed from the spray nozzles 8a and put in gas-liquid contact with the exhaust gas 1. The SOx in the exhaust gas reacts with calcium compounds in the absorbing liquid and converted to calcium sulfite (including calcium bisulfite), which is an intermediate product, drops to the reservoir 4a of the absorbing tower 4, is oxidized to gypsum and thereby converted into a final product (gypsum) by air supplied by an oxidizing air blower 21 into the absorbing liquid of the absorbing tower 4.
By thus supplying air directly into the absorbing tower 4, the reaction of absorption of the SOx in the exhaust gas and the oxidization reaction of the calcium sulfite produced are made to proceed simultaneously to promote the overall reaction and improve desulfurization performance. In addition, the oxidizing air supplied to the absorbing tower 4 in this process is made into microscopic bubbles by an oxidizing agitator 26 that agitates the absorbing liquid inside the reservoir 4a to improve usage efficiency of the oxidizing air.
The absorbing liquid is thereafter extracted from the reservoir 4a by an extracting pump 9 in accordance with an amount of gypsum produced, and a portion thereof is fed to a pH meter tank 30 and a pH of the absorbing liquid is measured by a pH meter 31 installed in the pH meter tank 30. The remaining portion of the absorbing liquid is fed to a gypsum dehydration system 10 and recovered as powder gypsum 11.
Meanwhile, water 12, separated at the gypsum dehydration system 10, is reused inside the gypsum dehydration system as water supplied to the limestone slurry tank 15, etc., and a portion thereof is extracted as wastewater 14 for preventing concentration of chlorine, etc., and fed to a waste water treatment system 50. At the wastewater treatment system 50, a chemical process by addition of a chemical or treatment by an ion adsorption resin, etc., and a biological process by bacteria are performed to perform a process of eliminating hazardous substances in the wastewater 14 so that amounts of respective components in the wastewater 14 fall below emission standards.
Each of FIGS. 4 and 5 shows structural diagrams of the absorbing tower 4 and the entrance flue 3 according to the conventional art described above. FIGS. 4A and 5A are both plan views and FIGS. 4B and 5B are both side views.
The absorbing tower 4 has a configuration where a large-diameter tank unit 13 and a small-diameter absorption unit 19 are joined together by a conical member 20, and whereas with the example shown in FIG. 4, which is disclosed in U.S. Pat. No. 5,656,046, the entrance flue 3 is disposed at the small-diameter absorption unit 19, with the example shown in FIG. 5, which is disclosed in FIGS. 3 and 4 of Japanese Patent No. 3549484 and U.S. Pat. No. 6,488,899, the entrance flue 3 is disposed at the conical member 20 that joins the large-diameter tank unit 13 and the small-diameter absorption unit 19. In addition, FIG. 5C is a perspective view of just the conical member 20.
[Patent Document 1] U.S. Pat. No. 5,656,046
[Patent Document 2] Japanese Patent No. 3549484
[Patent Document 3] U.S. Pat. No. 6,488,899